In London to promote his latest film, Avengers: Age of Ultron, Robert Downey Jr discussed superheroes with Channel 4 News presenter Krishnan Guru-Murthy.

But when the questions turned to his past experience of serving a prison sentence (for drug offences), the atmosphere cooled dramatically.

He declined to explain a comment in an interview with the New York Times some years ago that “you can’t go from a $2,000-a-night suite at La Mirage to a penitentiary and really understand it and come out a liberal,” saying it made sense at the time: “I could pick that apart for two hours and be no closer to the truth than giving you some half-arsed answer right now.

“I couldn’t even tell you what a liberal is”, he added.

On being asked if he minded further personal questions, Downey says “you have as much time as anyone else has.”

But when asked about his relationship with his father (the film star Robert Downey Senior) and the role it played in the dark periods he went through – the drug-taking and drinking – and whether he feels he’s free of all of that now, Downey Jnr shook his head, says “I’m sorry, I really don’t .. what are we doing?” and walked out on the interview.